This invention relates to variable focal length lenses. There is a need for such lenses as spectacle lenses due to a well known effect of aging on the human eye. As a result of aging, the lens in the human eye becomes less elastic and incapable of sufficient accommodation to focus on near or intermediate objects. For this reason, by age forty five most people require spectacles to achieve clear vision at close or intermediate distances. After the onset of the condition of limited focal accommodation, called presbyopia, a single set of fixed focus spectacles will be found to be unsatisfactory for both distant and near vision, irrespective of the wearer's general visual acuity. Whatever correction (if any) may be required to correct a person's vision for distance, an additional amount of optical power (up to three or four diopters) will be found to be required to correct that person's eyesight for near vision. The required "near addition" generally does not involve an astigmatic component.
The usual solution to this problem is to fit persons suffering from presbyopia with spectacles having bifocal lenses. In the most common form of bifocal lens, the upper part is ground to provide the wearer with the proper correction (if any is needed) for distance vision, and the lower part is ground with the same correction, plus a relatively small near addition. The near addition generally does not exceed a few diopters of optical power. Using bifocal lenses in a pair of spectacles allows a person to see distant objects clearly by looking straight ahead, and to see close objects clearly by looking downward.
The bifocal solution is not entirely satisfactory for at least three reasons. Firstly, many people have difficulty in adapting to bifocals; secondly, there is often the need to see near objects which are on a level with, or above, the wearer; and finally, there is usually a mid distance at which neither part of the lens is satisfactory. Trifocals or progressive multifocal lenses are used to help alleviate this latter problem, but the first two difficulties remain, and in addition, the in-focus field of view may be more limited than is desired.
There have been many attempts to devise a variable focal length lens which could be used in spectacles, but until the present invention, none has been practical. The concept of using a flexible fluid filled lens to achieve a variable focal length dates back to about 1890, but despite much effort since, no practical lens has been devised. The early work in this area was described in an article appearing in the November, 1940 issue of the Journal of the Optical Society of America, pp 560-563. The author of this article discussed the prior designs as being of two general types. The first employed two thin spherical glass segments whose edges were held together by a circumferential ring, and the enclosed space filled with a clear liquid. To change the focal length of the lens, the ring was contracted diametrally by squeezing the glass segments with a constricting or sphincter-like action, thereby altering the curvature of the lens surfaces. The second type of variable focal length lens described consisted of a sandwich of two thin spaced flat sheets with a clear liquid filling. Forcing additional liquid into the sandwich caused bowing of the sheets and varied the focal length of the lens. One problem common to both of these prior art designs, according to the author of the article, was that they resulted in distorted lens surfaces.
Examples of more recent attempts to devise a practical variable focal length lens for spectacle use are described in U.S. Pat. Nos. 3,598,479, 3,614,215, and 4,261,655. None proved successful. It is noted that the area of the variable focal length portion of the lenses in both the '479 and '655 patents has been severely restricted. The inventor of the '479 patent refers to the practical necessity of storing and pumping sufficient liquid to make the system functional as limiting the feasible diameter of his variable focal length lens. The same limitation would apply to the lens described in the '655 patent. Another shortcoming (applicable to all three examples) is that the liquid is of necessity ducted or piped through one or more joints in the spectacle assembly, creating the likelihood of leaks. Finally, the use of glass as a flexible membrane as disclosed in the '479 patent, because of its limited strain capability, also limits the size of the lens.
The present invention overcomes the deficiencies mentioned above, and provides a long sought practical and economical solution to the problem of varying the focal length of a spectacle lens.